This invention relates to infrared-transparent structures, and, more particularly, to such structures having therein a polymeric layer such as an adhesive or a surface coating.
Multicomponent infrared-transparent structures are used in a variety of infrared devices. The structures may be of many types, such as, for example, lenses and windows. Such structures must have a high optical transparency and low distortion in the infrared, and also exhibit required mechanical and physical properties such as strength, resistance to distortion and damage arising from thermal mismatches, good thermal dissipation, and low mass. Additionally, the structures must be readily producible.
One of the applications of most interest for multicomponent infrared-transparent structures is multicomponent lenses such as doublets or triplets. Two, three, or more different optical lenses are joined together in a facing relationship to make a compound lens. For many applications, such a multicomponent lens may be designed to have better optical and mechanical properties than a single-component lens. Multicomponent lenses may be constructed with the lenses joining only at their peripheries, leaving an air gap between the lenses.
In theory, the multiple elements might instead be cemented together with a continuous layer of an adhesive lying between the pairs of components, so that the adhesive lies in the optical path. For such cemented-doublet designs, the adhesive must have high transmissivity and low light distortion in the infrared. It must also adhere well to the lenses which it joins and impart good mechanical properties to the assembly.
Cemented doublets are known for visible-light-transparent lenses, but to date they have not been known for infrared-transparent lenses. The cemented design has optical and mechanical properties which are, in theory, superior to those of air-gap designs, but infrared-transparent polymer adhesives of sufficiently good optical and mechanical quality have not been available to make practical the cemented designs. Accordingly, there is a need for such a polymeric material that may be used as a cement or adhesive between infrared-transparent optical components. Although the problem has been described in respect to the problem of multicomponent lenses, it is also of concern for other applications such as infrared-transparent films and surface layers.
The present invention provides an infrared-transparent structure having excellent infrared transmission properties, particularly in the 3.6-7 micrometer wavelength range, and also excellent mechanical and physical properties. The approach is applicable to infrared-transparent multicomponent structures and also to infrared-transparent articles with surface films and layers. The present approach promotes good mechanical stability of joined optics, optical designs with greater power, improved thermal dissipation, reduced thermal stresses, and reduced distortion due to temperature changes. Total internal reflection angles are reduced, providing higher performance optical designs. Additionally, optical alignment is improved, system mass is reduced for many cases, and there is improved producibility.
In accordance with the invention, an infrared-transparent structure comprises a first infrared-transparent element having a first face, most preferably where the element is transparent to infrared energy in the wavelength range of from about 3.6 to about 7 micrometers (the xe2x80x9c3.6-7 micrometer wavelength rangexe2x80x9d). The structure further includes a layer of a solvent-free polymer material disposed on the first face of the first infrared-transparent element. For the preferred case where the element is transparent in the 3.6-7 micrometer wavelength range, the polymer material is characterized by an infrared transparency wherein a 10-micrometer thickness of the polymer material is at least 95 percent transparent (exclusive of Fresnel losses) to infrared energy in the 3.6-7 micrometer wavelength range.
In a preferred case, the polymer comprises an addition-cured silicone such as addition-cured dimethyl silicone, diphenyl silicone, or methylphenyl silicone, and also comprises an adhesion promoter. The adhesion promoter is preferably 3-glycidoxypropyltrimethoxysilane. The adhesion promoter is present in an effective amount, preferably from about 1 to about 4 percent by weight of the total of the polymer and the adhesion promoter. The layer of polymer is from about 0.0002 to about 0.002 inches thick.
In one embodiment that is of particular interest, the structure further includes a second infrared-transparent element having a second face disposed in facing relationship to the first face of the first-infrared-transparent element. The polymer is disposed between and contacts the first face and the second face and serves as an adhesive or cement holding the first and second elements together. This infrared optical doublet forms a compound lens that has excellent optical, mechanical, and physical properties. The structure may be extended further with the bonding of additional lenses, to form triplets and even-more-complex lenses.
The present approach is particularly useful where the first face of the first infrared-transparent element is coated with an antireflective coating such as one including a zinc sulfide top layer, and where the second face of the second element, where present, is similarly provided with an antireflective coating. The use of the adhesion promoter serves to achieve good bonding between the polymer layer and the antireflective coating.
The most preferred polymer layer composition, addition-cured dimethyl silicone and 3-glycidoxypropyltrimethoxysilane in an amount of from about 1 to about 4 percent by weight of the total, is particularly useful because it has no solvent to form distorting bubbles or the like in the mixture if trapped, produces no volatile byproducts during curing, and has low outgassing during and after curing. It has relatively low viscosity so that thin films may be formed, and it may be vacuum degassed to remove entrapped gasses. The addition-curing operation produces no byproducts which must be removed from the thin interfacial layer which, if not removed, result in distortions in the layer. The cured polymer has a low elastic modulus so that it can accommodate thermal expansion mismatches between joined components or when serving as a film.
The polymer is prepared by mixing the constituents, degassing as necessary, and applying the mixture as a thin layer. The layer is used at a free surface or as an adhesive to join another element to the first. The polymer is thereafter cured, preferably by heat curing.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.